1. Field of the Invention
The present invention relates to a mobile telephone/portable telephone system (cellular system) using a direct spread code division multiple access (DS-CDMA) scheme and, more particularly, to a cellular system characterized by a transmission method using pilot channels for coherent detection and transmission power control.
2. Description of the Prior Art
As a conventional technique of the present invention, an IS-95 system as a standard cellular system in North America is available.
In a forward link (base station→mobile terminal) according to the IS-95 specifications, a pilot channel commonly used for channels for all mobile terminals is prepared, and signals are constantly transmitted by using about 20% of the base station transmission power. On the pilot channel, a single spread code having a relatively large length (a 215 chip period=about 26.6 ms) is used, and a non-modulated signal (i.e., normally “0”) is transmitted. The reception section of a given mobile terminal estimates the transmission path of the radio signal received by using this pilot channel (estimates the delay, phase, and amplitude). This transmission path is used to determine the de-spreading timing of a data channel addressed to the given mobile terminal and perform coherent detection and RAKE combining.
Another conventional technique is a wideband CDMA scheme (to be referred to as a W-CDMA hereinafter), which has not been put into practice. The W-CDMA is a scheme that has been studied for the third-generation cellular system (IMT-2000). Currently, in Japan, IMT-2000 standards proposals are being prepared by the Association of Radio Industries and Businesses (ARIB).
According to a conventional proposed W-CDMA scheme, a known pilot symbol (since this data is time-multiplexed with another data, it is called a pilot symbol instead of a pilot channel unlike in IS-95) is added to a channel directed to each mobile terminal.
In IS-95, a pilot channel is prepared in only a forward link. In the conventional W-CDMA, pilot channels are prepared for the respective data channels, and hence a pilot symbol is also added to a reverse link (mobile terminal→base station). The reception sections of a mobile terminal and a base station estimate a transmission path by using this pilot symbol as in IS-95, and the estimated transmission path is used to demodulate data through a data channel addressed to itself.
A pilot symbol is added to each data channel in W-CDMA in this manner to use an efficient coherent detection scheme by estimating a transmission path using the same method in both a reverse link and a forward link, thereby improving the reverse link quality. Another purpose of the addition of such pilot symbols is to reduce radio wave radiation in unnecessary directions by changing the directivity of the base station antenna for each mobile terminal in a forward link so as to improve the forward link quality.
This method is called an adaptive antenna (adaptive array antenna) or smart antenna technique. This technique is a kind of space division multiple access (SDMA) in terms of reuse of radio waves upon space division. In a CDMA cellular system, in which radio waves having the same frequency are used in all the cells, the SDMA is regarded as a promising future technique.
The first problem in the prior art is that the method of preparing a common pilot channel to all terminals as in IS-95 described above does not match well with the technique of controlling transmission directivity for each mobile terminal by using an adaptive antenna array.
That the antenna directivity of the base station changes for the respective mobile terminals means that a pilot channel through which reference signals are uniformly transmitted in all terminal directions differs in transmission paths from a data channel through which information is transmitted by an antenna whose directivity is focused on the self-terminal, and the transmission path estimation result obtained by using the pilot channel cannot be used to demodulate the information through the data channel. For example, some of multipath channels detected by using the pilot channel may fall outside the data channel directivity range. In addition, there is no guarantee that the carriers are in phase.
The second problem in the prior art is that in the method of adding a pilot symbol to each channel as in the conventional W-CDMA scheme, the overheads of the pilot symbols become excessively large, resulting in poor transmission efficiency, especially in speech communication at a low data rate.
In the conventional W-CDMA scheme, four pilot symbols are transmitted at 0.625-ms intervals, which can be regarded as an overhead corresponding to 4.26 kbps in consideration of error correction code efficiency=⅓. This overhead is not small as compared with a data rate for high-efficiency speech, e.g., 8 kbps.
The third problem in the prior art is that when a pilot symbol is added to each data channel, since large power cannot be assigned, a high-quality reference signal cannot be obtained, although about 20% of the total transmission power of the base station can be assigned to obtain a high-quality reference signal when a common pilot channel is used as in IS-95 described above. The reference signal with poor quality must be improved by, for example, filtering. This influences the complexity of each terminal.
An adaptive array antenna is a future technique and hence should not be used in consideration of cost in the early stage of introduction. It is preferable that investment in equipment be made without any adaptive array antenna in the early stage of introduction, and investment in this technique as an improved technique is made with an increase in traffic.
Transmission path estimation does not depend on data rates. For this reason, when a high data rate is set, e.g., when data services are offered at 384 kbps, the overhead of a pilot symbol can be neglected. When, therefore, low-speed voice services are mainly offered, the common pilot channel scheme is advantageous, and the individual pilot channel scheme will become an indispensable technique in the future regardless of whether high-speed data services are mainly offered. Therefore, there are demands for a flexible scheme capable of smoothly coping with changes in services in this manner.